Structural polarization conversion in integrated optical vertically stacked ring resonators

In this paper we report the structural polarization conversion effect occurring in an integrated optics device formed by two vertically stacked ring resonators excited through an underlying bus waveguide. We demonstrate that the vertical propagation of light, due to evanescent coupling, is enhanced by the resonant behavior of the device and the polarization state of a horizontally polarized input wave tends to be rotated within the device. In particular, a gradual polarization rotation can be observed when passing from one propagation plane to another, due to the geometry of the structure. This effect has been explained by taking into account all the physical mechanisms, which contribute to the polarization conversion. Although numerical results of general validity have been obtained, we also considered, as an example, silicon nitride technology due to its intrinsic features related to low cost and reduced technological problems.

► We examined the physical mechanisms contributing to the polarization conversion. ► We analyzed the e.m. field distribution in a vertically coupled waveguides structure. ► The polarization state of a wave horizontally polarized is rotated by the device. ► We designed the structure to minimize all the polarization conversion sources. ► The residual polarization conversion can be exploited for polarization converters.